It is known in the art to incorporate metals such as thallium or arsenic into electrolyte solutions for depositing gold from alkali metal gold sulphite complexes from aqueous electroplating solutions. The use of such metal additives is stated to enhance the brightness of the deposit as well as to improve its finish and grain size.
U.S. Pat. No. 3,562,120 describes a gold metal electrolytic process with a bath containing a minor amount of thallium, calculated as metal. The pH of the electrolyte solution was within the range of 3 to 6. A later patent, U.S. Pat. No. 3,644,184, calls for an electrolyte solution which is neutral or alkaline and which contains gold in the form of an alkali metal gold cyanide complex. The pH is disclosed as being at least 6.5 and preferably from about 7 to about 13. A variety of acids may be added to the electroplating solution to achieve the desired pH. These acids include weak organic acids such as formic acid, citric acid, acetic acid, tartaric acid, gluconic acid, and the like. The thallium is added in the form of a water soluble salt such as thallous and thallic salts including the sulfates, nitrates, sulphide, chlorides, fluosilicates, and the like.
In a later patent, U.S. Pat. No. 3,666,640, the novel electroplating bath is prepared from an alkali metal gold sulphite complex. Along with the use of various other additives including certain metal additives, the patentee states that the addition of small amounts of arsenic, antimony or selenium can be utilized to improve the hardness of the gold metal deposit. The use of chelating agents such as disodium EDTA compounds, nitro and amino polycarboxylic acids, and hydroxy organic acids such as citric acid, lactic acid and tartaric acid is also disclosed.
The use of arsenic as an additive in combination with a carboxylic acid in electroplating bath solutions is known from U.S. Pat. No. 3,776,822. In that patent the gold is utilized in the form of an alkali metal gold sulfite complex, and according to the patentee the combination of the foregoing components in the electroplating bath provides gold deposits with controlled hardness values below 130 Knoop. The metals which may be added to the bath include arsenic, antimony, selenium as well as tellurium. These are provided in the form of their soluble salt. The polycarboxylic acids employed by the patentee include succinic, malonic and oxalic acids as well as their derivatives such as maleic acid. The preferred combination of a polycarboxylic acid and "semi-metal additive" is oxalic acid and arsenic trioxide, respectively. However, the use of arsenic as an additive has certain disadvantages in that it readily oxidizes from the trivalent state to the pentavalent state at which time its usefulness as a brightener/grain refiner ceases. Furthermore, the control of such electroplating bath solutions is very difficult. Conventional analytical procedures only determine the total arsenic content of the bath and do not distinguish between the active trivalent state and the inactive pentavalent state. Thus, despite the fairly developed state of this art there is still a problem to be solved insofar as it would be desirable to have an easily analyzable system where the alkali metal gold sulphite eletrolyte consistently produces a pure, bright, soft gold deposit.
In summary, gold metal deposits from non-cyanide complexes such as alkali metal gold sulphites tend to be hard, e.g. 140 Knoop, when using either thallium or arsenic salts as brighteners/grain refiners. Without these grain refiners, the gold deposits tend to be powdery and of little use to the electronics industry. Nevertheless, the hardness of the gold deposits with either thallium or arsenic metal additives give deposits having hardnesses unacceptable to the semiconductor industry, which generally requires a gold metal purity of about 99.9% and a hardness value below 90 Knoop.